Apparatus and method for thermal dissipation in a thermal mass flow sensor

ABSTRACT

A thermal mass flow controller includes a sensor assembly, a valve assembly, and a mass flow controller housing. A thermally conductive element conducts heat from the valve assembly away from the sensor assembly, thereby reducing uncontrolled contributions of heat to the sensor assembly.

RELATED APPLICATIONS

Patent applications entitled, “Method and Apparatus For ThermalIsolation Of A Thermal Mass Flow Sensor”, having inventors, JesseAmbrosina and Isao Suzuki, (Attorney Docket No. MKS-92) and “Apparatusand Method For Thermal Management of A Mass Flow Controller”, havinginventors, Jesse Ambrosina, Isao Suzuki, and Ali Shajii, (AttorneyDocket No. MKS-94), assigned to the same assignee as this applicationand filed on even date herewith are hereby incorporated by reference intheir entirety.

FIELD OF THE INVENTION

The present invention relates to mass flow measuring systems and, moreparticularly, to mass flow sensor housings that substantially eliminatethermal gradients that might otherwise be imposed upon a mass flowsensor.

BACKGROUND OF THE INVENTION

Capillary tube thermal mass flow sensors exploit the fact that heattransfer to a fluid flowing in a laminar tube from the tube walls is afunction of mass flow rate of the fluid, the difference between thefluid temperature and the wall temperature, and the specific heat of thefluid. Mass flow controllers employ a variety of mass flow sensorconfigurations. For example, one type of construction involves astainless steel flow tube with two or more resistive elements inthermally conductive contact with the sensor tube. The resistiveelements are typically composed of a material having a high temperaturecoefficient of resistance. Each of the elements can act as a heater, adetector, or both. One or more of the elements is energized withelectrical current to supply heat to the fluid stream through the tube.If the heaters are supplied with constant current, the rate of fluidmass flow through the tube can be derived from temperature differencesin the elements. Fluid mass flow rates can also be derived by varyingthe current through the heaters to maintain a constant temperatureprofile.

Such thermal mass flow sensors may be attached to a mass flowcontroller, with fluid from the controller's bypass tube feeding thecapillary tube (also referred to herein as the sensor tube). Since massflow measurements are dependent upon the temperature effects of thefluid upon the resistive elements, any external differential temperatureimparted to the resistive elements could produce errors in themeasurement of a mass flow rate. Unfortunately, thermal mass flowsensors are frequently operated in environments where an externalthermal gradient might be imposed upon them. For example, a thermal massflow sensor may be operated in close proximity to a valve coil thatdissipates significant power as it operates. Heat generated fromoperation of the valve coil may be communicated through a conductivethermal path, such as that provided by a mass flow controller housing,to the mass flow sensor. The heat thus-communicated may impose a thermalgradient upon the mass flow sensor housing which could, in turn,superimpose the external thermal gradient upon the sensor's resistiveelements, thus leading to errors in mass flow measurements.

A mass flow sensor that substantially eliminates externally imposedthermal gradients would therefore be highly desirable.

SUMMARY OF THE INVENTION

A mass flow controller in accordance with the principles of the presentinvention includes a sensor assembly, a valve assembly, and a mass flowcontroller housing to which the valve and sensor assemblies areattached. A thermally conductive element conducts heat from the valveassembly away from the sensor assembly, thereby reducing uncontrolledcontributions of heat to the sensor assembly. In an illustrativeembodiment, the mass flow controller includes a thermally conductiveenclosure that substantially envelops the sensor and valve assemblies.The thermally conductive element substantially surrounds and makesthermally conductive contact with the valve assembly while, at the sametime, making substantial conductive thermal contact with the enclosure.

In an embodiment that does not include the enclosure, the thermallyconductive element may include structure on one or more surfaces thataccelerate the dissipation of thermal energy from within the valveassembly. Such structure may include fins located on one or moreexterior surfaces of the thermally conductive element that do not facethe sensor assembly. The thermally conductive element may be composed ofa high thermal conductivity material, such as aluminum. In anillustrative embodiment, the thermally conductive element is integral tothe valve assembly. That is, in this illustrative embodiment, theexterior wall of the valve assembly is formed to conduct thermal energyaway from the mass flow controller sensor assembly.

These and other advantages of the present disclosure will become moreapparent to those of ordinary skill in the art after having read thefollowing detailed descriptions of the preferred embodiments, which areillustrated in the attached drawing figures. For convenience ofillustration, elements within the Figures may not be drawn to scale.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a sectional drawing of a mass flow controller in accordancewith the principles of the present invention that employs heatsink thatprovides a thermally conductive path between the mass flow controllervalve assembly and a thermally conductive mass flow controllerenclosure;

FIG. 2 is a partial conceptual block diagram and partial sectionaldrawing of a mass flow rate sensor assembly in accordance with theprinciples of the present invention;

FIG. 3 is a perspective view of the exterior of a mass flow ratecontroller in accordance with the principles of the present invention;and

FIG. 4 is a perspective view of the exterior of a mass flow controllerin accordance with the principles of the present invention, whichincludes a thermally conductive enclosure.

DETAILED DESCRIPTION OF DISCLOSURE

A mass flow controller in accordance with the principles of the presentinvention includes a mass flow sensor assembly and a valve assembly. Inan illustrative embodiment, the mass flow sensor assembly is attached tothe mass flow controller housing and includes a heat sink that isattached to the valve assembly housing and provides a low thermalimpedance path to the surrounding atmosphere in order to dissipate heatgenerated through operation of the valve within the valve assembly.

The sectional view of FIG. 1 illustrates the major components of a massflow controller in accordance with the principles of the presentinvention. The mass flow controller 100 includes a thermal mass flowsensor assembly 102 and a valve assembly 104. The valve assembly 104 isconnected to the mass flow controller housing 108 to control the rate offlow of gas in response to control signals generated by a mass flowsensor circuitry described generally in the discussion related to FIG.2. The mass flow controller 100 includes an inlet 106 for receiving aflow of gases to be metered. The process gas enters the mass flowcontroller though the inlet 106 and travels through the valved opening110 to a bypass channel 112. The valve 114 operates under control of themass flow sensor and related circuitry to admit a precisely measuredquantity of process gas into the inlet port 106, through the controller,and out the outlet port 116 for a processing application, such as may beemployed in integrated circuit manufacturing. The bypass channel 112 isconnected to the inlet port 106 to receive and carry the stream of gas.

A laminar flow element 118 rests within the channel 112 and provides apressure drop across the thermal mass flow sensor assembly 102 anddrives a portion of the gas through the sensor tube 120 of the thermalmass flow sensor 102. The thermal mass flow sensor assembly 102 includescircuitry that senses the rate of flow of gas through the controller 100and controls operation of the valve assembly 114. The thermal mass flowsensor assembly 102 is attached to a wall 122 of the mass flowcontroller that forms a boundary of the bypass channel 112. Input 124and output 126 apertures in the wall 122 provide access to the mass flowsensor assembly 102 for a gas travelling through the mass flowcontroller 100. In this illustrative embodiment the mass flow sensorassembly 102 includes a baseplate 128 for attachment to the wall 122.The baseplate 128 may be attached to the wall and to the remainder ofthe thermal mass flow sensor assembly 102 using threaded hole and matingbolt combinations, for example. Input 130 and output 132 legs of thesensor tube 120 extend through respective input 134 and output 136apertures of the baseplate 128 and, through apertures 124 and 126, enterthe mass flow controller wall 122.

The mass flow sensor assembly includes top 138 and bottom 140 sectionsthat, when joined, form a thermal clamp 141 that holds both ends of thesensor tube active area (that is, the area defined by the extremes ofresistive elements in thermal contact with the sensor tube) atsubstantially the same temperature. The thermal clamp also forms achamber 142 around the active area of the sensor tube 120. That is, thesegment of the mass flow sensor tube 102 within the chamber 142 has, inthermal communication with it, two or more resistive elements 144, 146,each of which may act as a heater, a detector, or both. One or more ofthe elements is energized with electrical current to supply heat to thefluid as it streams through the tube 120.

The thermal clamp 141, which is typically fabricated from a materialcharacterized by a high thermal conductivity relative to the thermalconductivity of the sensor tube, makes good thermally conductive contactwith the portion of the sensor tube just downstream from the resistiveelement 144 and with the portion of the sensor tube just upstream fromthe resistive element 146. The thermal clamp thereby encloses andprotects the resistive elements 144 and 146 and the sensor tube 120.Additionally, the thermal clamp 141 thermally “anchors” those portionsof the sensor tube with which it makes contact at, or near, the ambienttemperature. In order to eliminate even minute errors due to temperaturedifferentials, the sensor tube may be moved within the thermal clamp toinsure that any difference between the resistance of the two coils isdue to fluid flow through the sensor tube; not to temperature gradientsimposed upon the coils from the environment.

A heat sink 150 is attached in good thermal communication with thehousing 152 of the valve assembly 104. Power dissipated by a solenoid inoperation of the valve 114 may generate a significant amount of heatthat might travel through the mass flow controller housing 108 andsuperimpose an external thermal gradient upon the sensor assembly 102.Such an external thermal gradient superimposed on the sensor resistiveelements 144 and 146 would create an error in the determination of massflow through the sensor tube. That is, since the thermal mass flowsensor assembly 102 relies upon the tendency of the fluid flowingthrough the sensor tube 120 to establish a thermal gradient to measurethe mass of fluid flowing through the tube 120, an externally imposedthermal gradient would render a false reading. In this illustrativeembodiment, the heat sink 150 substantially envelopes the valve assemblyhousing 152 and conducts heat away from the valve assembly housing 152.

In an embodiment in which the heat sink 150 is open to the atmosphere,features such as fins 151 may be added to the heat sink to increase thetransport of heat away from the valve assembly housing 152 and away fromthe sensor assembly 102. That is, fins may be added to one or more sidesof the heat sink 150 not facing the sensor assembly 102 in order toincrease convective and radiant heat transport away from both the sensorassembly 102 and the valve assembly 104. Alternatively, in anillustrative embodiment described in the discussion related to FIGS. 3and 4, the heat sink 150 extends to make thermally conductive contactwith a controller housing 164 that encloses the thermal mass flow sensor102 and valve 104 assemblies. In such an embodiment, a smooth-surfacedheat sink 150 provides greater surface contact with the controllerhousing and thereby provides a more substantial thermally conductivepath for dissipation heat from the valve assembly 104. A mass flowcontroller in accordance with the principles of the present inventionsuch as presented in the illustrative embodiment of FIG. 3 employs botha heat sink 150 and thermal ground 148 to substantially eliminatethermal gradients that might otherwise impair operation of the thermalmass flow sensor assembly 102.

FIG. 2 illustrates in greater detail an embodiment of a mass flow sensorin accordance with the principles of the present invention. The bypasstube 112, laminar flow element 118, upstream resistive element 146 anddownstream resistive element 144 are as previously described. Thearrangement of the thermal clamp 141, including top 138 and bottom 140portions, and its thermally conductive communication with the sensortube 120 is illustrated in greater detail here. The broken lines of thesensor tube 120 indicate that the tube is substantially surrounded andin conductive thermal contact with the thermal clamp 141 around itsentire circumference at each end of its operational segment. Theoperational segment of the sensor tube is defined for illustrativepurposes as that segment of the sensor tube disposed between theupstream 154 and downstream 156 legs of the thermal clamp.

One end of the bypass tube 112 defines an input port 119 and the otherend of the bypass tube 112 defines an output port 121 so that fluid mayflow from the input port 119 to the output port 121 in a downstreamdirection indicated by arrows 123. The laminar flow element 118 isdisposed within the bypass tube 112 for restricting the flow of fluidthrough the bypass tube 112. An upstream end of the sensor tube 120couples to the bypass tube between the input port and the laminar flowelement. A downstream end of the sensor tube 120 couples to the bypasstube 112 between the laminar flow element 118 and the output port 121, afixed proportion of the total mass of fluid flowing from the input port119 to the output port 121 flows through the sensor tube 120. The sensortube 120 may be of capillary dimensions and is fabricated from amaterial, such as steel, that is characterized by a relatively highthermal conductivity in comparison to the thermal conductivity of thefluid.

Each of the resistive elements 144 and 146 includes a thermallysensitive resistive conductor that is wound around a respective portionof the sensor tube 120, each successive turn of the conductors beingplaced close to the previous turn. Each of the resistive elementsextends along respective portions of the sensor tube 120 along an axisdefined by the operational segment of the sensor tube 120. Downstreamresistive element 144 is disposed downstream of the resistive element146. The elements abut one another or are separated by a small gap formanufacturing convenience and are electrically connected at the centerof the tube. Each resistive element 144, 146 provides an electricalresistance that varies as a function of its temperature. The temperatureof each resistive element 144, 146 varies as a function of theelectrical current flowing through its resistive conductor and the massflow rate within the sensor tube 120. In this way, each of the resistiveelements 144, 146 operates as both a heater and a sensor. That is, theelement acts as a heater that generates heat as a function of thecurrent through the element and, at the same time, the element acts as asensor, allowing the temperature of the element to be measured as afunction of its electrical resistance. The mass flow sensor 102 mayemploy any of a variety of electronic circuits, typically in aWheatstone bridge arrangement, to apply energy to the resistive elements146 and 144, to measure the temperature dependent resistance changes ineach element 144, 146 and, thereby, the mass flow rate of fluid passingthrough the tube 120. Circuits employed for this purpose are disclosed,for example, in U.S. Pat. No. 5,461,913, issued to Hinkle et al and U.S.Pat. No. 5,410,912 issued to Suzuki, both of which are herebyincorporated by reference in their entirety.

In operation, fluid flows from the input port 119 to the output port 121and a portion of the fluid flows through the restrictive laminar flowelement 118. The remaining fluid flows through the sensor tube 120. Thecircuit (not shown) causes an electrical current to flow through theresistive elements 144 and 146 so that the resistive elements 144 and146 generate and apply heat to the sensor tube 120 and, thereby, to thefluid flowing through the sensor tube 120. Because the upstreamresistive element 146 transfers heat to the fluid before the fluidreaches the portion of the sensor tube 120 enclosed by the downstreamresistive element 144, the fluid conducts more heat away from theupstream resistive element 146 than it does from the downstreamresistive element 144. The difference in the amount of heat conductedaway from the two resistive elements is proportional to the mass flowrate of fluid within the sensor tube 120 and, by extension, the totalmass flow rate through the mass flow rate controller 100 from the inputport 119 through the output port 121. The circuit measures thisdifference by sensing the respective electrical resistances andgenerates an output signal that is representative of the mass flow ratethrough the sensor tube 120.

The baseplate 122 may be integral to the thermal mass flow sensorassembly 102 or it may be attached to the assembly 102 through any of avariety of attachment means, such as threaded through-holes and bolts,for example. The baseplate 122 is configured to provide a thermal pathbetween the sensor assembly 102 and the remainder of the mass flowcontroller 100 and to thereby maintain the average temperature of thesensor assembly 102 and the remainder of the mass flow controller atsubstantially the same average temperature.

The perspective view of FIG. 3 provides an external view of the sensorassembly and valve assembly as previously described. The valve assemblyincludes a heatsink 150 that is formed to provide a substantial thermalpath between the valve assembly 104, the heatsink 150 and an enclosure164 to be described in the discussion related to FIG. 4. The inlet 106and outlet 116 ports are as previously described. The sensor assembly102 includes top 138 and bottom 140 thermal clamp components attached tothe wall of the controller assembly through the baseplate 128. Thebaseplate 128 is held in position with bolts 160. Mass flow controllerelectronics (not shown) are attached to the controller with a metalflange 162. Exterior surfaces of the heatsink 150 make conductivecontact with the interior surfaces of a controller enclosure. In thisillustrative embodiment, at least three exterior surfaces of theheatsink make thermally conductive contact with the thermally conductiveenclosure; which three exterior surfaces of the heatsink will beapparent from examination of FIG. 4.

The perspective view of FIG. 4 illustrates a view of a mass flowcontroller assembly in accordance with the principles of the presentinvention in which a thermally conductive enclosure 164 envelopes thevalve assembly 104 and thermal mass flow sensor assembly 102, aspreviously described. A heatsink that makes conductive thermal contactwith the valve assembly may also make thermal contact with the enclosure164. The enclosure may also incorporate features, such as fins, toaccelerate heat exchange between the enclosure's interior and exterior.

While there have been illustrated and described particular embodimentsof the present disclosure, it will be appreciated that numerous changesand modifications will occur to those skilled in the art. Accordingly,it is intended that the appended claims cover all those changes andmodifications which fall within the true spirit and scope of the presentdisclosure.

What is claimed is:
 1. A thermal mass flow controller comprising: athermal mass flow controller housing including a fluid input port and afluid output port and a bypass channel disposed between the input andoutput ports; a thermal mass flow sensor assembly mounted to the thermalmass flow controller housing for measuring a flow of fluid through thebypass channel; a valve assembly mounted to the thermal mass flowcontroller housing for controlling the flow of fluid through the bypasschannel; and a heat sink device thermally coupled to the valve assemblyfor conducting thermal energy from the valve assembly.
 2. The thermalmass flow controller of claim 1 wherein the heat sink device includes amass which substantially surrounds the valve assembly.
 3. The thermalmass flow controller of claim 2 wherein the valve assembly is mounted tothe thermal mass flow controller housing proximate the fluid input port.4. The thermal mass flow controller of claim 3 wherein the thermal massflow sensor assembly is mounted proximate the fluid output port.
 5. Thethermal mass flow controller of claim 4 wherein a greater portion of themass is disposed on a side of the valve assembly opposite the thermalmass flow sensor assembly.
 6. The thermal mass flow controller of claim4 further including an enclosure device coupled to the thermal mass flowcontroller housing and enclosing the thermal mass flow sensor assemblyand the valve assembly therein, the enclosure device contacting the heatsink device thereby forming a conductive thermal path from the heat sinkdevice to an area external to the enclosure device through the enclosuredevice.
 7. The thermal mass flow controller of claim 2 wherein a greaterportion of the mass is disposed on a side of the valve assembly oppositethe thermal mass flow sensor assembly.
 8. The thermal mass flowcontroller of claim 7 wherein said greater mass is positioned on theheat sink device such that the thermal energy from the valve assembly isconducted away from the thermal mass flow sensor assembly.
 9. Thethermal mass flow controller of claim 2 wherein the heat sink deviceincludes fins mounted to a surface thereof for accelerating theconduction of thermal energy from the valve assembly.
 10. The thermalmass flow controller of claim 9 wherein the fins are positioned on theheat sink device such that the thermal energy from the valve assembly isconducted away from the thermal mass flow sensor assembly.
 11. Thethermal mass flow controller of claim 2 further including an enclosuredevice coupled to the thermal mass flow controller housing and enclosingthe thermal mass flow sensor assembly and the valve assembly therein,the enclosure device contacting the heat sink device thereby forming aconductive thermal path from the heat sink device to an area external tothe enclosure device through the enclosure device.